Squirty Star Imitates Black Hole

Squirty Star Imitates Black Hole

Scientists using CSIRO's Australia Telescope near Narrabri in
northern NSW have made a discovery that they hope will increase
our understanding of a fundamental cosmic process.

The team has seen a neutron star spitting out a jet of matter
at very close to the speed of light. This is the first time such
a fast jet has been seen from anything other than a black hole.

The discovery, reported in this week's issue of the journal Nature,
challenges the idea that only black holes can create the conditions
needed to accelerate jets of particles to extreme speeds.

"Making jets is a fundamental cosmic process, but one that
is still not well understood even after decades of work," says
team leader Dr Rob Fender of the University of Amsterdam.

"What we've seen should help us understand how much larger
objects, such as massive black holes, can produce jets that we
can see half-way across the Universe."

The scientists, from The Netherlands, the UK and Australia, studied
Circinus X-1, a bright and variable source of cosmic X-rays, over
a three-year period.

Circinus X-1 lies inside our Galaxy, about 20 000 light-years
from Earth in the constellation Circinus near the Southern Cross.

It consists of two stars: a 'regular' star, probably about 3 to
5 times the mass of our Sun, and a small compact companion.

"We know that the companion's a neutron star from the kind
of X-ray bursts it's been seen to give off," says team member
Dr Helen Johnston of the University of Sydney.

"Those X-ray bursts are a sign of a star that has a surface.
A black hole doesn't have a surface. So the companion must be a
neutron star."

A neutron star is a compressed, very dense ball of matter formed
when a giant star explodes after its nuclear fuel runs out. In
the hierarchy of extreme objects in the Universe, it is just one
step away from a black hole.

The two stars in Circinus X-1 interact, with the neutron star's
gravity pulling matter off the larger star onto the neutron star's
surface.

This 'accretion' process generates X-rays. The strength of the
X-ray emission varies with time, showing that the two stars of
Circinus X-1 travel around each other in a very elongated orbit
with a 17 day period.

"At their point of closest approach, the two stars are almost
touching," says Dr Johnston.

Since the 1970s astronomers have known that Circinus X-1 produces
radio waves as well as X-rays. A large 'nebula' of radio emission
lies around the X-ray source. Within the nebula lies the new-found
jet of radio-emitting material.

Jets are believed to emerge, not from black holes themselves,
but from their 'accretion disk' - the belt of dismembered stars
and gas that a black hole drags in towards it.

In Circinus X-1 it's likely that the accretion disk varies with
the 17-day cycle, being at its most intense when the stars are
at their closest point in the orbit.

The jet from Circinus X-1 is travelling at 99.8% of the speed
of light. This is the fastest outflow seen from any object in our
Galaxy, and matches that of the fastest jets being shot out of
other complete galaxies. In those galaxies, the jets come from
supermassive black holes, millions or billions of times the mass
of the Sun, that lie at the centres of the galaxies.

Whatever process accelerates jets to near the speed of light,
it does not rely on the special properties of a black hole.

"The key process must be one common to both black holes and
neutron stars, such as accretion flow," says Dr Kinwah Wu,
formerly of the University of Sydney, and now at Unversity College
London in the UK.